Despite usually taking beautiful photos that you can't wait to share with all your Facebook friends or Instagram followers, some albums are just way too private for the all world the world to see.
Imagine going to a party and having a crazy fun time with your friends. Or going on a wonderful vacation with your partner. You surely don't want just anyone to see the pics, but you want your friends and partner to have access to them.
Cluster allows you to create different groups (one for your family, one for friends, one for you and your partner) and share your private photo albums (as well as notes and files!) with them.